As lithium secondary batteries are widely used ranging from small-sized electronic devices to electric vehicles, power storage devices, and the like, there is an increasing demand for a cathode material for a secondary battery having high safety, a long service life, high energy density and high output characteristics.
In this regard, a lithium-excess layered structure lithium metal composite oxide is a cathode active material having a high capacity of 240 mAh/g or more per unit weight, and has been highlighted as a cathode material for a next-generation electric vehicle and power storage, which requires high capacity characteristics.
However, the lithium-excess layered structure lithium metal composite oxide has a high irreversible capacity caused by the phase change during the first charge and discharge so that it is difficult to implement a high discharge capacity, and is disadvantageous in that the service life thereof is rapidly decreased due to the elution of manganese ions and side reactions with electrolyte at high temperature. Further, there is a risk that the lithium-excess layered structure lithium metal composite oxide may be reacted with an electrolyte at high temperature to cause fire or explosion when used in a lithium secondary battery due to the structural instability occurring in a high charge state.
Accordingly, in order to implement a cathode material composed of a lithium-excess layered structure composite oxide, which reduces an initial irreversible capacity of the lithium-excess layered structure composite oxide, has excellent high rate capability, and may be used for a long time, attempts have been made to suppress side reactions with an electrolyte and enhance structural stability by coating various materials on a surface of a cathode active material.
Among them, it is known that a coating with metal/metal oxide/metal fluoride, and the like enhances electrochemical characteristics of the cathode active material, and particularly, it is reported that a coating of a metalloid fluoride such as AlF3 greatly enhances electrochemical characteristics of a layered structure compound (see Non-Patent Document 1, and the like).
In particular, Patent Document 1 discloses a technology that prevents service life characteristics of a battery, particularly, performances thereof from deteriorating at high pressure and high rate by wet-coating a surface of a cathode active material for a lithium secondary battery with a finely powdered fluorine compound. In addition, Patent Document 2 briefly discloses that a metal fluoride or a metalloid fluoride may be used in a coating by using a solution based precipitation approach using an aqueous solvent while referring to Patent Document 1.
Furthermore, Patent Document 2 introduces that “the use of metal oxide or metal phosphate coatings has been described in published U.S. patent application 2006/0147809 to Amine et al. (the '809 application), entitled “Long Life Lithium Batteries with Stabilized Electrodes,” incorporated herein by reference. Specifically for active materials with a spinel or olivine crystal structure, the '809 application specifically describes coatings comprising ZrO2, TiO2, WO3, Al2O3, MgO, SiO2, AlPO4, Al(OH)3, or mixtures thereof”.
Meanwhile, the inventors of Patent Document 1 describe in Patent Document 3 that as a coating is performed by spraying a coating composition for a surface treatment while a lithium-containing compound is heated, a structural transfer occurring on the surface of a cathode active material finally obtained is prevented, while indicating a problem of a wet coating method of a metal fluoride disclosed in Patent Document 1, and the like as in “since a powder of a coating-treated cathode active material forms aggregates so that a particle size distribution is changed, and the composition and structure of the surface of the cathode active material are changed due to excessive contact with water or an organic-based solvent, the coating effect is not exhibited 100%” or “lithium in excess is easily dissolved in water or an organic solvent and thus causes a structural transfer of the cathode active material to reduce electrochemical characteristics” when the coating is performed by a wet coating method as in Patent Document 1.
Further, the inventors of Patent Document 1 disclose in Non-Patent Document 1 that when coated on the surface of a cathode active material, a metal fluoride and a metal oxyfluoride protect the cathode active material from hydrofluoric acid present in an electrolyte to maintain the crystal structure of the cathode active material well, and increase the drift velocity of lithium ions from the electrolyte to the cathode active material to decrease an increase in internal resistance.